Thyrotoxicosis is known to induce a broad range of changes in carbohydrate metabolism. Recent studies have identified the sympathetic and parasympathetic nervous system as major regulators of hepatic glucose metabolism. The present study aimed to investigate the pathogenesis of altered endogenous glucose production (EGP) in rats with mild thyrotoxicosis. Rats were treated with methimazole in drinking water and L-thyroxine (T₄) from osmotic minipumps to either reinstate euthyroidism or induce thyrotoxicosis. Euthyroid and thyrotoxic rats underwent either a sham operation, or a selective hepatic sympathetic (Sx) or parasympathetic denervation (Px). After 10 days of T₄ administration, all animals were submitted to a hyperinsulinemic euglycemic clamp combined with stable isotope dilution, to measure EGP. Plasma tri-iodothyronine (T₃) showed a fourfold increase in thyrotoxic as compared with euthyroid animals. EGP was increased by 45% in thyrotoxic as compared with euthyroid rats and correlated significantly with plasma T₃. In thyrotoxic rats, hepatic PEPCK mRNA expression was increased 3,5-fold. Relative suppression of EGP during hyperinsulinemia was 34% less in thyrotoxic than in euthyroid rats, indicating hepatic insulin resistance. During thyrotoxicosis, Sx attenuated the increase in EGP, while Px resulted in increased plasma insulin with unaltered EGP as compared with intact animals, compatible with a further decrease in hepatic insulin sensitivity. We conclude that chronic, mild thyrotoxicosis in rats increases EGP, while it decreases hepatic insulin sensitivity. Sympathetic hepatic innervation contributes only to a limited extent to increased EGP during thyrotoxicosis, while parasympathetic hepatic innervation may function to restrain EGP in this condition.